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Chapter 26. Local Anesthetics

A 67-year-old woman is scheduled for elective total knee arthroplasty. What local anesthetic agents would be most appropriate if surgical anesthesia were to be administered using a spinal or an epidural technique, and what potential complications might arise from their use? What anesthetics would be most appropriate for providing postoperative analgesia via an indwelling epidural or peripheral nerve catheter?

Simply stated, local anesthesia refers to loss of sensation in a limited region of the body. This is accomplished by disruption of afferent neural traffic via inhibition of impulse generation or propagation. Such blockade may bring with it other physiologic changes such as muscle paralysis and suppression of somatic or visceral reflexes, and these effects might be desirable or undesirable depending on the particular circumstances. Nonetheless, in most cases, it is the loss of sensation, or at least the achievement of localized analgesia, that is the primary goal.

Although local anesthetics are often used as analgesics, it is their ability to provide complete loss of all sensory modalities that is their distinguishing characteristic. The contrast with general anesthesia should be obvious, but it is perhaps worthwhile to emphasize that with local anesthesia the drug is delivered directly to the target organ, and the systemic circulation serves only to diminish or terminate its effect. Local anesthesia can also be produced by various chemical or physical means. However, in routine clinical practice, it is achieved with a rather narrow spectrum of compounds, and recovery is normally spontaneous, predictable, and without residual effects. The development of these compounds has a rich history (see section: Historical Development of Local Anesthesia), punctuated by serendipitous observations, delayed starts, and an evolution driven more by concerns for safety than improvements in efficacy.

Historical Development of Local Anesthesia

Although the numbing properties of cocaine were recognized for centuries, one might consider September 15, 1884, to mark the "birth of local anesthesia." Based on work performed by Carl Koller, cocaine's numbing effect on the cornea was demonstrated before the Ophthalmological Congress in Heidelberg, ushering in the era of surgical anesthesia. Unfortunately, with widespread use came recognition of cocaine's significant CNS and cardiac toxicity, which along with its addiction potential, tempered enthusiasm for this application. As the early investigator Mattison commented, "the risk of untoward results have robbed this peerless drug of much favor in the minds of many surgeons, and so deprived them of a most valued ally." As cocaine was known to be a benzoic acid ester, the search for alternative local anesthetics focused on this class of compounds, resulting in the identification of benzocaine shortly before the turn of the last century. However, benzocaine proved to have limited utility due to its marked hydrophobicity, and was thus relegated to topical anesthesia, a use for which it still finds limited application in current clinical practice. The first useful injectable local anesthetic, procaine, was introduced shortly thereafter by Einhorn, and its structure has ...